NASA’s planet-hunting Kepler spacecraft has been crippled by the failure of one of the reaction
wheels that keep it pointed, according to astronomers close to the situation.

If engineers cannot restore the wheel or find some other way to keep the spacecraft’s telescope
pointed, it could spell a premature end to one of the most romantic and successful of NASA’s
missions: the search for Earthlike planets in habitable orbits around other stars.

Just last month, astronomers reported that Kepler had found two planets only slightly larger
than Earth orbiting in the “Goldilocks” zone, where liquid water is possible, of a star 1,200
light-years away.

More planet candidates, even smaller and closer to being Earthlike, lurk in the pipeline,
astronomers say, but they have not yet been confirmed.

Kepler was launched in March 2009 to determine the fraction of stars in the galaxy that harbor
Earthlike planets by surveying 150,000 stars in the constellations of Cygnus and Lyra, looking for
the dips in starlight caused by planets passing, or transiting, in front of their suns.

The spacecraft has identified 115 planets and has a list of 2,740 other candidates.

Since Earth transits only once a year, two more years would have given astronomers a chance to
see more transits of the planets they are looking for. Without the extra time, the answer will be a
little more uncertain than it might have been.

“It was one of those things that was a gift to humanity,” said one astronomer who spoke on
condition of anonymity because NASA had not yet made the news public. “We’re all going to lose for
sure.”

In January, engineers noticed that one of the reaction wheels that keep the spacecraft pointed
was experiencing too much friction. They shut the spacecraft down for a couple of weeks to give it
a rest, in the hopes that the wheel’s lubricant would spread out and solve the problem. But when
they turned it back on, the friction was still there.

Until now, the problem had not interfered with observations, which are scheduled to go on until
at least 2016.

Kepler was launched with four reaction wheels, but one failed last year after showing signs of
erratic friction. Three wheels are required to keep Kepler properly and precisely aimed.